In a typical fiber optic transmission network, optical frequency modulation (FM) characteristics of communication laser sources such as laser diodes must be accurately determined in order to define the limits within which information can be reliably modulated and exchanged between optical terminals in the network. The optical frequency behavior of a particular laser source under modulation is determinative of the ability of that laser source to transmit information. When a laser source is directly modulated with an input current signal, the laser source output exhibits a frequency deviation (.DELTA.f) per milliAmpere which varies as a function of the laser input modulation frequency. This FM characteristic is commonly referred to as the laser chirp. For most optical systems, an accurate characterization of laser chirping is essential to ensure transmission accuracy and improve performance. For example, in wavelength division multiplex (WDM) networks, laser chirp measurements are used to fine tune the laser operating wavelengths for an optimal use of the available optical bandwidth thereby enhancing transmission performance.
Presently, laser chirp is measured with specialized equipment that typically includes sophisticated network or optical spectrum analyzers. For network applications using multiple laser sources, the use of this equipment is not cost-effective. In a typical WDM network for example, many laser sources are used in different transmitting terminals which necessitates the duplication of the chirp measuring equipment used for each transmitting location. Duplicating this equipment for each transmitting location in the network may very rapidly prove to have a major impact on the cost associated with chirp characterization.
There may be situations where it is practical to use the same equipment for laser sources in different locations. In these cases, it would also be desirable to have chirp-measuring equipment that is simple and more portable. This is due to the fact that the chirp measurement equipment currently used is heavy and bulky which makes it difficult to transport between transmitting sites.